Typically, plasma reactors use a radio frequency (RF) power source with a constant average power or voltage to excite a plasma in a vacuum chamber. Plasma reactors in which the RF power source is coupled to the process chamber inductively, also referred to as inductively coupled plasma (ICP) reactors, are widely used, for example, in silicon and metal etch applications. Most of these reactors have an additional RF generator coupled proximate the substrate in which plasma is coupled capacitively to the chamber. This additional RF generator is often referred to as a bias RF generator. Commonly used frequencies for both source and bias in ICP reactors for the semiconductor industry include 13.56 MHz.
The inventors have observed that, due to the nature of the coupling between the source and bias, such ICP reactors can almost independently control the plasma density via the source RF power and the ion energy via the bias RF power. However, some RF coupling between the two sources still takes place if the bias power is high enough to alter the plasma properties dictated mainly by the source. Such coupling could cause instabilities in the plasma due to slight differences in frequencies generated by the respective generators (for example, due to crystal variations). To overcome this deficiency, a common exciter may be used to force both generators to generate the same single frequency.
Unfortunately, however, the inventors have observed another potential problem that may arise due to phase misalignment of the signals produced by the respective generators. For example, although current commercial ICP reactors try to align both source and bias signals to have zero phase difference, the actual phase difference is rarely, if ever, zero. In addition, variations in the actual phase between source and bias generators naturally exist, causing chamber-to-chamber variation in their respective phase differences. Such differences in phase between chambers affects the ability to provide consistent processing amongst otherwise identical chambers.
Therefore, the inventors have provided improved inductively coupled plasma reactors as described herein.